CHEMICAL CONSTITUENTS OF THE) SOIL 159 



from soils kill wheat seedlings at dilutions of 100 parts per 

 million, and are injurious in lower amounts. 1 Fertilizers decrease 

 the injurious effects of certain organic substances. 2 



Dauheney/' at Oxford, England, tested the old toxic theory of 

 De Condalle by a rotation experiment, in which 18 different crops 

 were grown continuously on the same plots in comparison with 

 the same crops grown in various rotations ; the yields were not 

 sufficient to justify the assumption of the existence of a toxin. 

 Russell 4 grew six crops of rye in succession on sand containing 



1 ! t 



J 3 



Fig. 37. Wheat seedlings ten days old grown in water containing di- 



hydroxystearic acid (i) 200 parts per million ; (2) 100 parts; (3) 50 



parts ; (4) 20 parts ; (5) o parts. Bureau of Soils. 



nutrient salts. A seventh crop grown on this sand, and another 

 crop on fresh sand, were practically equal in size. Other experi- 

 ments with rye on soil, and with buckwheat and spinach on sand 

 and on soil, give practically the same results. Hence there is no 

 evidence that the previous crops left toxic residues. 



1 Bulletin 47. 



2 Bulletin 70. 



3 Phil, Trans., 1845, p. 179. 



4 Soil Conditions and Plant Growth, p. in. 



